A change in shoes saw Novak Djokovic lose his footing at times in the semifinals but didn't stop him from defeating Juan Martin del Potro. (Jonathan Brady/AFP/Getty Images)

WIMBLEDON, England -- First Roger Federer, now Novak Djokovic. During his fantastic five-set win over Juan Martin del Potro in the Wimbledon semifinals, Djokovic was slipping and sliding and hitting the turf with more frequency than his previous matches. At first it looked as though it was Del Potro's big hitting that was keeping him on the run and guessing, but as it turns out, it actually might have been Djokovic's footwear.

The Telegraph reports the All England Club asked Djokovic to change his Adidas shoes after his quarterfinal win over Tomas Berdych because the nubs on his shoes gave him an unfair advantage. The Grand Slam Handbook states that grass court shoes "with pimples or studs around the outside of the toes shall not be permitted."

Photographs show that Djokovic's shoes had pimples on sides of his toebox, which would have given him more traction when he slides on the outside of his foot. Tournament referee Andrew Jarrett asked Djokovic to file down the nubs, which he did before the semifinals.

One well-known former player told Telegraph Sport on Friday that he was not surprised that the referee’s office had intervened.

“When you look at how Novak moves into that splits position, and then pushes off with his toes, the pimples on the front of the shoe come into play.

“Other players were interested in what happened with those shoes.

"If Djokovic had been given the all clear, we would have seen more people using them, because this is a sport of tiny margins.

"A fraction more balance and speed on one movement could be enough to make the difference.”

"My shoes are fine," Djokovic said on Saturday during his pre-final interview. "When you're defending, especially the back of the court, it's a little bit more slippery because there is a little bit more grass. So there is not much grip, and that's why you slip. My adjustments are made. I'm fully fit for what's coming up."

It's the second shoe-related controversy of the tournament after the All England Club asked Federer to abandon his orange-soled Nikes because they were deemed to violate the club's "predominantly white" dress code. Federer changed his shoes after the first round and then lost to Sergiy Stakhovsky in his next match.